Iraqi security forces launch a rocket against Islamic State militants positions during clashes in the western side of Mosul, Iraq, Sunday, March 5, 2017. (AP Photo/Khalid Mohammed)

(Newser)
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A US bomb that dropped on a building in the Iraqi city of Mosul set off explosive materials that had already been placed inside by Islamic State fighters, causing the structure to collapse and killing more than 100 civilians, a US military investigation concluded Thursday. The 500-pound bomb dropped by a US aircraft on March 17 was intended to kill two ISIS snipers who posed a threat to Iraqi counterterrorism forces, said the lead investigator, US Air Force Brig. Gen. Matthew Isler. The probe found that the US bomb triggered secondary explosions from devices clandestinely planted in the lower floors of the concrete building, Isler said, per the AP.

Isler said neither the Iraqi troops nor the Americans who authorized and conducted the airstrike knew civilians were in the building or that the explosive materials were present. It was likely the largest single incident of civilian deaths since the US air campaign against ISIS began in 2014, and represents about a quarter of all civilian deaths in that time. Isler said 101 civilians in the building were killed, and another four died in a nearby building. He said 36 civilians remain unaccounted for.